Democrats hoped this presidential election would be a cakewalk. In their eyes, the presumptive Republican nominee, Donald Trump, spent most of the spring alienating big chunks of the electorate, beginning with women. Meanwhile, the presumptive Democratic candidate, Hillary Clinton, has run a careful, well-funded, well-honed campaign. What could go wrong?

And yet, in a spate of reputable surveys Trump has suddenly erased the advantage Clinton had held all year. The average of major polls compiled by the website RealClearPolitics shows the two candidates tied with 43 percent each. In at least three polls, Trump has even pulled ahead by a slim margin. That’s been enough to send some Democrats into a swivet. But they shouldn’t panic.

First of all, polls in May don’t have much predictive value about an election that’s more than five months away.

Four years ago, in May 2012, Mitt Romney was tied with President Barack Obama in the RealClearPolitics average, just as Trump and Clinton are tied today. In November, Romney lost by four percentage points.

In May 2008, John McCain was only a little behind Obama, according to the same index. In November, McCain lost by seven percentage points.

Or go all the way back to May 1980, when then-President Jimmy Carter held a forbidding 12-point lead over Republican challenger Ronald Reagan in one survey. In November, Reagan won by a wide margin

There’s one big reason Trump is doing better than expected in the polls: Republican voters have rallied behind him faster than some analysts expected.

In an NBC News-Wall Street Journal poll released this week, Trump’s support among Republicans in a head-to-head contest with Clinton jumped to 86 percent, up from 72 percent a month earlier.

Democrats, by contrast, are still divided between Clinton and Bernie Sanders. In the same poll, only about two-thirds of Sanders voters were willing to say they would vote for Clinton in November if she wins the nomination. Her support among Democrats has actually declined slightly, from 87 percent a month ago to 84 percent now, and so has her backing among independents.

That depresses Clinton’s overall number and makes her look weak, but it’s probably only a temporary weakness.

If history is any guide, almost all of those Sanders voters will move into Clinton’s column once she is actually the nominee, especially if Sanders keeps his promise to campaign for Clinton (or against Trump) in the fall.

One more factor that should help Clinton: Obama has regained some of his lost popularity. In recent polls, the president’s job approval has reached 51 percent, its highest level since his second inauguration in 2013. That means Obama would be an asset to the Democratic nominee in the fall.

All that said, Clinton still faces serious challenges.

After six years of political gridlock, many voters are yearning for change in Washington, and that should help Trump. In the NBC-Journal poll, 53 percent of voters said they would prefer a president who would bring major changes to Washington, “even if it is not possible to predict what the changes may be.” Only 43 percent said they’d prefer “a steady approach (with) fewer changes.”

Clinton also has a problem with independent and moderate voters, two different but not always distinct categories. Several polls have shown her losing to Trump among independents. Among moderates, who ought to be a natural constituency for her, the NBC-Journal poll found her running roughly even with the Republican.

Finally, a problem both candidates face: They’re the least-popular candidates ever to win their parties’ nominations. Both are viewed unfavorably by a majority of voters — the first time that has happened in the history of modern polling.

Democrats and Republicans alike appear driven this year by what political scientists call “negative partisanship.” They may not like their standard-bearer much, but they loathe the other side’s candidate with a passion.

The polls that show Trump’s support rising and Clinton’s slipping are neither mirages nor predictions. They’re snapshots of a moving target. It’s too early to panic. But it’s never too early to worry.

Doyle McManus is a columnist for the Los Angeles Times.